Biomechanics of the cervical spine Part 2. Cervical spine soft tissue responses and biomechanical modeling

Anatomy for surgeons

Study design A finite element model of the ligamentous cervical spinal segment was used to compute loads in various structures in response to clinically relevant loading modes. Objective To predict biomechanical parameters, including intradisc pressure, tension in ligaments, and forces across facets that are not practical to quantify with an experimental approach. Summary of background data Finite element models of the cervical spine in their present form, because of inherent assumptions and simplifications, are not entirely satisfactory for studying the biomechanics of the intact, injured, and stabilized cervical spinal segment. Methods A three-dimensional finite element model of a C5-C6 motion segment was developed from serial computed tomographic scans of a ligamentous cervical spinal segment. This model included nonlinear ligament definition, fully composite intervertebral disc, fluid nucleus, and Luschka's joints. The model-based displacement predictions were in agreement with the experimental data. This model was used to predict load sharing and other related parameters in spinal elements in response to various loading modalities. Results In axial compression, 88% of the applied load passed through the disc. The interspinal ligament experienced the most strain (29.5%) in flexion, and the capsular ligaments were strained the most (15.5%) in axial rotation. The maximum intradisc pressure was 0.24 MPa in the flexion with axial compression mode (1.8 Nm + 73.6 N). The anterior and posterior disc bulges increased with the increase in axial compression (up to 800 N). Conclusions The results provide new insight into the role of various elements in transmitting loads. The model represents significant and essential advancement in comparison with previous finite element models, making it possible for such models to be used in investigating a broad spectrum of clinically relevant issues.

Prediction of load sharing among spinal components of a C5-C6 motion segment using the finite element approach.

Biochemistry of the Intervertebral Disc

Many experimental testing techniques have been adopted in order to provide an understanding of the biomechanics of the human intervertebral disc (IVD). The aim of this review article is to amalgamate results from these studies to provide readers with an overview of the studies conducted and their contribution to our current understanding of the biomechanics and function of the IVD. The overview is presented in a way that should prove useful to experimentalists and computational modellers. Mechanical properties of whole IVDs can be assessed conveniently by testing ‘motion segments’ comprising two vertebrae and the intervening IVD and ligaments. Neural arches should be removed if load-sharing between them and the disc is of no interest, and specimens containing more than two vertebrae are required to study ‘adjacent level’ effects. Mechanisms of injury (including endplate fracture and disc herniation) have been studied by applying complex loading at physiologically-relevant loading rates, whereas mechanical evaluations of surgical prostheses require slower application of standardised loading protocols. Results can be strongly influenced by the testing environment, preconditioning, loading rate, specimen age and degeneration, and spinal level. Component tissues of the disc (anulus fibrosus, nucleus pulposus, and cartilage endplates) have been studied to determine their material properties, but only the anulus has been thoroughly evaluated. Animal discs can be used as a model of human discs where uniform non-degenerate specimens are required, although differences in scale, age, and anatomy can lead to problems in interpretation.

https://research-information.bris.ac.uk/files/106096608/1_s2.0_S1751616117300449_main.pdf

Biomechanics of the human intervertebral disc: a review of testing techniques and results

Posterior and anterior heights, cross-sectional area and shape were measured for all the intervertebral discs in four spines from elderly human cadavers. Disc height was a minimum at the T4–5 level; thoracic discs were less wedge-shaped than those in the cervical and lumbar regions. Cross-sectional area increased from the cranial to caudal extremity; at the L5-S1 level the nucleus pulposus occupied a high proportion of this area. Cervical discs tended to have an elliptical cross-sectional shape, thoracic discs were more circular and lumbar discs tended to have an elliptical cross-section which was flattened or re-entrant posteriorly. This shape distribution was quantified by defining a shape index which had a maximum value of 1 for a circular cross-section. Orientations of the reinforcing fibres in the outer lamellae of the anterior annulus fibrosus were measured from 27 discs by X-ray diffraction. For these measurements, C3–4, T7–8 and L2–3 were chosen as representative of cervical, thoracic and lumbar discs. The fibre tilt, with respect to the axis of the spine, was significantly less in the cervical discs (at 65°) than in the thoracic and lumbar discs (about 70°). These findings are interpreted in relation to differing functional requirements and possible mechanisms of failure in the cervical, thoracic and lumbar regions of the spine in the light of current knowledge on the biomechanics of the intervertebral disc.

J. S. Pooni

Papers

Comparison of the structure of human intervertebral discs in the cervical, thoracic and lumbar regions of the spine